1. Field of the Invention
The present invention relates to methods and compositions for initiating the reduction of the viscosity of fracturing fluids introduced into a subterranean formation and, more particularly, to such methods and compositions which are designed so that the reduction of the viscosity is initiated by closure of fractures in the subterreanean formation.
2. Setting of the Invention
Hydraulic fracturing treatments and fracture-acidizing treatments are commonly utilized to increase the permeability of a subterreanean formation for greater hydrocarbon flow from the formation to a wellbore. In such treatments, fracturing fluids are introduced into the subterranean formation under sufficient pressure to create cracks or fractures in the formation and to also propagate these fractures out into the formation. Generally, the fracturing fluids contain entrained proppants, such as sand or sintered bauxite, so that as the fracturing fluid seeps into the formation or is backflowed out from the fractures, the fractures close upon the proppants to maintain the fractures in an open state for increased permeability.
In utilizing certain fracturing fluids, such as high viscosity aqueous gels, water-hydrocarbon emulsions, or oil-based fluids, it is preferred to maintain the viscosity of these fracturing fluids while the fractures are being created and propagated, as well as to aid in transporting the proppants to the farthest reaches of the fractures. Ideally the viscosity of the fracturing fluids is maintained until the time at which the fractures close upon and trap the proppants, but before the proppants settle to the bottom of the fractures. After the proppants have been trapped in the fractures, it is desirable that the viscosity of the fluids be quickly reduced to allow the fluids to flow back through the fractures, around the proppants and back into the wellbore. The ultimate success of the treatment depends partly on the proper and quick removal of the fracturing fluids. Chemicals utilized to reduce the viscosity of fracturing fluids are commonly called "breakers" or "breaker fluids" and are introduced into the fractures to act immediately upon the fracturing fluids upon contact with the fluids or upon reaching a predetermined temperature. Breakers commonly used include oxidizing agents, enzymes, acids, catalysts of iron, copper and silver, and mixtures thereof. Specific examples of breakers include sodium persulfate and ammonium persulfate, alpha and beta amylases, amyloglucosidase, aligoglucosidase, invertase, maltase, cellulase, hemicellulase, fumaric acid, nitric acid, and the like. These breaker chemicals can be in the form of either a liquid or a powder which is activated by contact with oil or water. Premature chemical reaction of the breaker with the fracturing fluids can significantly degrade the fracturing fluids, i.e., reduce their viscosity before the proper termination of a treatment, which can significantly reduce the overall effectiveness of the treatment.
Various methods have been proposed to delay the viscosity-reducing action of the breakers by forming the breaker into pellets and then coating the breaker pellets. One such method is disclosed in U.S. Pat. No. 4,202,795 to Burnham, et al. In Burnham, pellets of a breaker fluid material are encapsulated within a gel coating, and included in the pellets is a gel-degrading substance. The pellets have a predetermined time delay after which the gel dissolving chemical will dissolve the protective gel coating to release the breaker chemical into the fracturing fluid to reduce its viscosity. A serious problem encountered when using this type of breaker system is that these systems tend to release their active ingredients over a significant period of time due to differences in the thickness of the protective coating and the length of time and temperature exposure of individual pellets introduced into the fluid system. Also, the hydraulic fracturing treatment can take much longer than anticipated, and the viscosity of the fracturing fluid is then reduced before the proppants are trapped in the furthest reaches of the fractures. If the hydraulic fracturing treatment takes a shorter period of time than anticipated, the operators must wait for the fracturing fluid's viscosity to be reduced so the treatment can be finished.
Another serious problem is in estimating the time after fluid introduction has stopped when the fracture will close and trap the proppants. This time interval can be from only a few minutes for high permeability formations or up to over 24 hours for very low permeability formations. If the proppants are not suspended in the farthest reaches of the fractures as the fracture closes, then the greatest effectiveness of the fracturing treatment generally has not been obtained; therefore, the greatest productivity of the well cannot be obtained. Also, if the operator needs to wait for the breaker fluid to reduce the viscosity of the fracturing fluid even after the fracture has closed upon the proppants, then this waiting or downtime can add substantial additional costs to the treatment.
There is a need for a method of using a breaker where the breaker is inert to the hydraulic fracturing fluid until activated by the closing of the fracture upon the proppants. In addition, there is a need for a method which uses much larger concentrations of breaker fluid than what can be normally utilized using the mentioned systems. This increased concentration of breaker fluid will not only permit the desired viscosity reduction to occur, but also greatly aid in the removal of organic residue of the viscosity and/or fluid loss agents generally used in the fracture fluids. The reduction of the residue can result in higher permeability for flow through the trapped proppants and hence result in a more effective fracture treatment.